1. Field of the Invention
The present invention relates to transmission apparatuses, and particularly to a transmission apparatus for performing signal transmission based on label switching while performing failure circumvention control.
2. Description of the Related Art
As the use of the Internet has been spreading, the traffic of Internet Protocol (IP) packets has been increasing in these days. The variety of contents in IP packets has also been increasing. Besides traditional computer data, an increasing amount of audio data, moving images, and other data that should be processed in real time are transmitted. Label switching in layer 2, or a technology for adding a destination label to an IP packet, receives attention under circumstances where the IP-packet transmission time should be reduced.
The label switching system is formally referred to as generalized multi-protocol label switching (GMPLS). This high-speed packet transfer technology allows different networks to be directly connected in layer 2 without using a higher layer, although internetworking was conventionally implemented in layer 3 by means of a router.
FIG. 39 shows an overview of GMPLS packet transmission. Nodes (label switch routers, or LSRs) 101 to 104 for passing a packet are connected in series by transmission media. The nodes 101 to 104 have individual label tables.
The label table contains input information IN and output information OUT. The input information IN includes an input interface identifier IFin, which is the identifier of a path to which a packet is input, and an input label Lin added to an input packet.
The output information OUT includes an output interface identifier IFout, which is the identifier of a path from which a packet should be output, and an output label Lout which should be added to an output packet. The nodes 102 and 103 have label tables ta and tb respectively, as shown in the figure.
Suppose that a packet is transferred from the node 101 to the node 104. The node 101 adds a label “a” to the packet and transfers the packet. The node 102 receives the packet having the label “a” from IFin#1 and searches through the label table ta for the corresponding output information OUT. The node 102 then replaces the label “a” with a label “b” in accordance with the output information OUT, and outputs the packet from IFout#2.
The node 103 receives the packet having the label “b” from IFin#1 and searches through the label table tb for the corresponding output information OUT. The node 103 then replaces the label “b” with a label “c” in accordance with the output information OUT, and outputs the packet from IFout#2. These steps are repeated until the packet reaches the ending node 104 in this example.
The GMPLS packet transfer performs high-speed packet transfer to a destination by determining an output interface in accordance with a label in a label table instead of an address and by rewriting the label added to the packet accordingly.
GMPLS is applied to time division multiplexing (TDM) transmission and wavelength division multiplex (WDM) transmission as well as packet transmission with a fixed-length label added at the beginning of a packet, as described above.
GMPLS applied to TDM does not add a label to data and uses a time slot as a label. In a synchronous optical network (SONET), for instance, channels are multiplexed by interchanging time slots, so that routing is determined by identifying the time slots.
GMPLS applied to WDM transmission uses an optical wavelength as a label. Switching is performed by identifying a wavelength as a label, and the optical signal is directly routed.
In an optical network where a rapidly increasing volume of communication traffic flows, a line disconnection, a node failure, or any other failure causes serious damage to service. Accordingly, a high-level failure recovery system is desperately needed. If a failure occurs in a GMPLS network, the working path is switched to a recovery-path so that the failure can be circumvented. This raises interest in flexible design of a recovery-path.
A conventional network failure circumvention system sends a path setup request message for the working path, containing the need or no need of protection, from the inlet node through the working path to the outlet node. When the response is sent back, the nodes automatically determine the route of a recovery-path. (See Japanese Unexamined Patent Application Publication No. 2002-344491 (paragraph numbers 0012 to 0023, FIG. 3), for instance).
The conventional system (disclosed in Japanese Unexamined Patent Publication No. 2002-344491) automatically routes the recovery-path, starting from a node on a working path when the working path is set up. If a plurality of recovery-paths is set up, the nodes cannot immediately establish a recovery-path best for the location of a failure because a section for determining the recovery-path to be selected is not provided.